Inverted nine-spot patterns are commonly used in steamflooding. Those patterns have a steam injection well at the center of the pattern, a production well at each of the four corners of the pattern, and a production well at the center of each side of the pattern. Steam is injected in the center well and oil is produced from the sidewells and corner wells.
In an inverted nine-spot pattern, the injector is closer to the side producer than the corner producer. If both producers are fully completed and the reservoir is areally homogeneous, steam breaks through to the sidewell first, delaying steam propagation toward the corner well. The result is that when a project reaches an economic limit, much oil remains unrecovered, especially in the lower part of the formation near the corner producer.
The effect of sidewell completion on steamflood performance in inverted nine-spot patterns has received little systematic evaltion. A previous simulation study by V. M. Ziegler ["A Comparison of Steamflood Strategies: Five-Spot Pattern vs. Inverted Nine-Spot Pattern," SPE Reservoir Engineering (Nov. 1987) 549-58] indicated that converting a five-spot pattern to an inverted nine-spot by drilling infill producers at the midpoints of the pattern boundaries increases and accelerates oil recovery. Partially completing the infill wells in the lower half of the drive zone was found to give higher oil recovery than that obtained by fully completing the sidewells. This study, however, did not consider the effect of sidewell completion on the performance of a steamflood pattern initially completed as an inverted nine-spot.
U.S. Pat. Nos. 4,166,501 and 4,177,752 disclose methods of improving vertical sweep in a five-spot pattern, but does not address the problem of producing oil from blind spots, such as sidewells.
U.S. Pat. No. 4,458,758 discloses well completion techniques when all producers are assumed to be at the same distance from the injector, but does not address the problem of balancing steam propagation when producers are at different distances.
U.S. Pat. Nos. 4,166,501, 4,177,752, and 4,458,758 are hereby incorporated by reference for all purposes.
SPE Paper 14337, "Infill Drilling in a Steamflood Operation: Kern River Field" discusses field experience with infill drilling which converts five-spot patterns to inverted nine-spots. It mentions no control of sidewell completions. Because steam has broken through to corner wells, there is no need to partially complete them.